Preheat your oven to 375 F. Grease a rimmed sheet pan with olive oil or line it with parchment paper. Make sure all corners and edges are well coated to prevent sticking.

Prepare your ingredients by chopping vegetables into small pieces. If using vegetables like mushrooms or onions, cook them briefly to remove excess moisture. Cut cooked meat into small pieces if using.

Crack the eggs into a large mixing bowl. Add milk, salt, and black pepper. Whisk thoroughly until the mixture is smooth and slightly frothy. This step helps create a light and fluffy texture.

Add the chopped vegetables, shredded cheese, and cooked meat into the egg mixture. Stir gently until everything is evenly distributed throughout the mixture.

Pour the mixture into the prepared sheet pan. Use a spatula or gently tilt the pan to spread the mixture evenly. Make sure ingredients are not clustered in one area.

Place the pan in the oven on the center rack. Bake for 18 to 22 minutes, or until the center is fully set and no longer jiggly. The top should be lightly golden.

Remove from the oven and allow the frittata to rest for 5 to 10 minutes. This helps it firm up and makes slicing easier.

Slice into squares or rectangles and serve warm. Store leftovers in the refrigerator and reheat as needed.